The present invention relates to an electric power supply unit which supplies the electric power to an engine control unit, and particularly to an electric power supply unit for the engine control unit which supplies the DC power to a computer for controlling an automobile engine.
Recently, the size of the semiconductor wafer for one microcomputer has become small from the viewpoint of the downsizing and the cost reduction. Moreover, an electric current increases if the clock speed goes up. Then, it is necessary to reduce the voltage to satisfy the electric power and reduce the entire electric power. The blocking voltage cannot be taken for the conventional voltage when the size of IC chip of the microcomputer becomes small like this and thus the blocking voltage has become lower. That is, a CPU core power unit has an inclination of adopting a lower voltage to decrease the loss when making the microcomputer speeded up.
On the other hand, the microcomputer needs a plurality of power units, because the reference voltage of an analog to digital converter and the digital I/O power unit voltage remain the conventional 5V voltage.
In the conventional electric power supply unit, 5V voltage is generated by the switching regulator to obtain the CPU core power supply voltage, and voltage 3.3V is generated by the series regulator to obtain the CPU core power supply voltage.
Further, 5V is generated from the voltage of the battery through 7.8V generation linear regulator as a reference voltage of the AD converter (For Instance, see pages 4-5 and FIG. 1 of Japanese Patent Application Laid-Open No. 11-265225).
In this official gazette, the countermeasure to decrease the regulator loss is done like this. However, in the microcomputer which requires a plurality of power supplies (For instance, when it is necessary to supply two of 5V and 3.3V voltages), The isolation in the microcomputer collapses when the voltage of two power supplies supplied to the microcomputer is reversed, and there is a possibility to cause latch-up.
Moreover, the blocking voltage of the elements used internally tends to become low by the shrink of the microcomputer in the electric power supply unit disclosed in the above official gazette. Therefore, these elements have potential of causing the blocking voltage breakdown when the potential difference between 5V and 3.3V power supplies is large.